This invention relates to a system for electrically stimulating the heart of a human or animal to improve the pumping action of the heart. The invention also relates to procedures and electrical leads for connecting a pulse generator (also referred to as a pacemaker or a cardiac stimulator) to a heart muscle. These procedures and leads permit a surgeon to implant electrical leads in both an atrium and ventricle of a heart through a single small incision in the chest.
The medical benefits of atrial-ventricular (A-V) sequential cardiac stimulation or pacing are well established. In A-V sequential pacing, electrical signals are applied to both atrial and ventricular regions of a heart muscle, and the two sets of signals are offset in time with respect to one another to produce properly sequenced electrical stimulation to the heart.
In spite of the recognized benefits of A-V sequential pacing, the use of this method of cardiac pacing has been somewhat limited by certain drawbacks. In the past, one important drawback has been that A-V sequential pacing requires that two sets of electrical leads be implanted in widely separated regions of the heart.
One method for implanting the two sets of leads is to open the chest in a formal thoracotomy. This method provides excellent access to the heart for lead placement, but the high surgical morbidity and mortality of formal thoracotomy is a severe drawback of the method. Patients with low cardiac reserve, who would most benefit from sequential pacing, would often not tolerate the proceedure.
A second approach for implanting the leads need for A-V sequential pacing is to introduce the leads transvenously into the heart. In this approach, leads are generally threaded through two blood vessels into the selected cardiac chambers.
In many cases, an atrial lead is passed transvenously into the atrium and is hooked in the atrial appendage. A ventricular lead is generally passed transvenously into the right atrium, through the tricuspid valve into the right ventricle. The transvenous approach to A-V sequential cardiac pacing generally requires the use of two veins and often difficult lead manipulation. Furthermore, proper atrial pacing is difficult to achieve with this approach, for the sensitivity of the atrium to electrical stimulation is not uniform and optimum atrial pacing can best be achieved by fixing the atrial electrode to that portion of the atrium most sensitive to stimulation. Patients subjected to previous cardiac surgery have greater difficulty with transvenous atrial pacing, because the loss of the atrial appendage makes the secure placement of the J-shaped electrodes almost impossible.
The transvenous approach avoids the high surgical morbidity and mortality of a formal thoracotomy. However, it brings with it other drawbacks. Undesired displacement of the lead is a common problem, and the optimal pacing site often cannot be selected because the lead cannot be secured to the appropriate portion of the atrial wall. Furthermore, there is an undesired incidence of perforation of the right ventricle with transvenous leads. This problem is exacerbated by the fact that it may be difficult to determine in positioning a transvenous lead whether resistance to lead movement is caused by impingement of the lead on the vessel wall or by the wall of the ventricle itself. Moreover, thrombosis of the subclavian vein has been reported in cases involving transvenous electrodes.